In many situations, air within a building must be continuously replaced, for health or comfort reasons. Conditions such as these are frequently found in paint spray shops, foundries, chemical plants, welding shops, large restaurants, bowling alleys, etc. However, taking in a large amount of ambient air can overburden building's heating system. In these situations, a “make-up” air heater is used to temper the incoming air, raising it to room temperature and thus relieving the building heating plant of the extra load.
In such situations, a gas manifold with a gas outlet is disposed in an air duct. The outlet is typically flanked on both sides by baffles with air inlets, defining therebetween a combustion chamber. The burner is located downstream of the airflow. Air flow, generated by a fan located downstream of the baffles, flows through the air inlets in the baffles and into the combustion chamber to mix with the gas and thus feed a flame. The baffles further serve to protect the flame from an excessive supply of air, thus preventing the flame from being quenched. The flame and its byproducts are mixed directly with the air stream and added to the space being heated. A heating process such as this does not require a heat exchanger and is therefore more energy efficient. However, the products of combustion, including carbon monoxide, nitrogen dioxide, and carbon dioxide, are not separated from the air stream, and are delivered directly to the occupied space.
Depending on the magnitude of the temperature change needed to the air, the burner firing intensity must be changed. The intensity changes from a minimum fire, in which the entire flame is maintained near the gas ports, to a high fire, in which the flame fills and in some cases exceeds and burns outside of the combustion chamber.
To accommodate the need for a dynamic firing intensity, the air inlets in the baffles are sized from very small next to the manifold, and increase progressively to the exit of the combustion chamber. When the flame is at low fire, only a small amount of air is necessary and the fire is maintained near the manifold. When the flame intensity increases, the flame fills the combustion chamber and is fed by the remaining openings located in the baffles.
In prior art designs, when the firing intensity is high, the flame is only established toward the end of the baffles away from the manifold. This is because only by the larger holes in the ends of the baffles is enough air admitted into the combustion chamber to create the proper air to fuel mixture. The flame can further extend outside of the protective baffles, exposing the flame to excess ambient air and thereby producing large amounts of nitrogen dioxide. If too much air is added to the combustion chamber at a particular firing rate, the flame is quenched, thereby resulting in high carbon monoxide emission.
Proper sizing and position of air openings in the burner baffles is therefore of importance. By sizing the openings and strategically placing them relative to the gas ports, flame characteristics can be shaped and controlled. The maintenance of high fire flame characteristics is also of high importance and has not heretofore been investigated. There is a need to size and place the baffle openings such that they can be utilized for controlling the flame shape and its characteristics throughout the entire firing rate, including high fire. Such control contributes to increased Btu output, a higher turndown ratio, flame stability and emission reduction.
Carbon monoxide and nitrogen dioxide emission levels are controlled by law. Currently, ANSI standards Z83.4, Non-Recirculating Direct Gas-Fired Industrial Air Heaters, and Z83.18, Recirculating Direct Gas-Fired Industrial Air Heaters dictate the emissions limits permitted by a direct-fired heaters. Moreover, not only are emission standards mandated, but it has been found that by lowering the emissions of carbon monoxide and nitrogen dioxide, overall performance of the burner can be increased. For example, lower carbon monoxide emissions permit the burner to operate in higher airflows, thereby increasing Btu output, while lower nitrogen dioxide emissions allow the burner and the air heaters to attain higher temperature rise, and thus increasing its operation range.
Moreover, many existing plants already have a manifold installed. Prior burners required a larger manifold and combustion chamber to increase Btu output. It would be of great benefit to retrofit an already installed manifold with baffles that increase Btu output while lowering emissions, without increasing the burner's footprint.